In recent years, the IEEE 802.11 standard for Wireless Local Area Networks (WLANs) has emerged as a prevailing technology for “last mile” broadband wireless access networks. While the principal application of these networks has been in providing connectivity to devices running best effort data applications, such as email and web browsing, interest is growing in supporting isochronous services such as telephony service or streaming video. Voice over Internet Protocol (VoIP) over WLAN (VoWLAN) is currently one of the fastest growing Internet applications.
In VoIP, the voice signal is sampled at a fixed rate. A “packet” is assembled from the data, including multiple headers, a payload and one or more trailers. The headers and trailers include: (i) information about the packet origin and destination; (ii) information about the packet's relative position in the stream of packets that comprise the call; (iii) information about the communications protocols being used by the network; and (iv) error-checking information, among other things. Taken together, the headers and trailers are referred to as the overhead associated with a packet. The payload is the portion of the packet that contains the voice samples corresponding to the digitized voice signal. The time between generation of two consecutive packets is known as the packetization interval. Typically, the packets are of a constant length, and the packetization interval is determined by the type of Vocoder being used to transmit the call. Typical vocoders use 10-30 ms packetization intervals. For example, G.711 type vocoders often use a 20 ms packetization interval.
For short packetization intervals, the fraction of a packet corresponding to overhead can be large, due mainly to the number of bytes required to characterize the various protocol layers. The transmission efficiency of the call is largely determined by the fraction of the packet that corresponds to the actual payload. Larger packets with a larger payload increase the efficiency of the communication, but under certain circumstances may result in poor quality of the phone call due to increased packetization latency.
In many cases, the devices used for VoIP calls are portable handheld devices that are powered by batteries. Short packetization intervals result in faster depletion of the stored energy in a battery-powered mobile communications device, because the percentage of time that the device is involved in transmitting/receiving of voice packets is higher for the same amount of data due to the overhead in each short packet. Longer packetization intervals are also desirable in this context to decrease power usage and conserve batteries.